add some documentation for gcc8

doc/source/faq.rst

@@ -39,6 +39,11 @@ The compilation stops while compiling Kokkos with ``/usr/include/stdlib.h(58): e
 When using the Intel compiler on a Mac Intel, I get a linking error involving the ``SharedAllocationRecordIvvE18t_tracking_enabledE`` symbol.
   This is a known bug of the Intel Mac compiler with Kokkos. Apparently Intel has decided not to fix it. Check the issue on the `Kokkos git page <https://github.com/kokkos/kokkos/issues/1959>`_.
 
+I get an error at link that says there are undefined symbols fo std::filesystem
+  This is a known bug/limitation of gcc8 that does not include filesystem extensions in the standard library.
+  You should try to clear up CMakeCache.txt and explicitely add the required link library when calling cmake as in
+  ``LDFLAGS=-lstdc++fs cmake $IDEFIX_DIR ...```
+
 Execution
 ---------
 

src/gravity/selfGravity.cpp

@@ -153,18 +153,18 @@ void SelfGravity::Init(Input &input, DataBlock *datain) {
 
   // Instantiate the bicgstab solver
   if(solver == BICGSTAB || solver == PBICGSTAB) {
-    iterativeSolver = new Bicgstab(*laplacian.get(), targetError, maxiter,
+    iterativeSolver = new Bicgstab<Laplacian>(*laplacian.get(), targetError, maxiter,
                                               laplacian->np_tot, laplacian->beg, laplacian->end);
   } else if(solver == CG || solver == PCG) {
-    iterativeSolver = new Cg(*laplacian.get(), targetError, maxiter,
+    iterativeSolver = new Cg<Laplacian>(*laplacian.get(), targetError, maxiter,
                                         laplacian->np_tot, laplacian->beg, laplacian->end);
   } else if(solver == MINRES || solver == PMINRES) {
-    iterativeSolver = new Minres(*laplacian.get(),
+    iterativeSolver = new Minres<Laplacian>(*laplacian.get(),
                                   targetError, maxiter,
                                   laplacian->np_tot, laplacian->beg, laplacian->end);
   } else {
       real step = laplacian->ComputeCFL();
-      iterativeSolver = new Jacobi(*laplacian.get(), targetError, maxiter, step,
+      iterativeSolver = new Jacobi<Laplacian>(*laplacian.get(), targetError, maxiter, step,
                                               laplacian->np_tot, laplacian->beg, laplacian->end);
   }